In the past, a wide variety of systems for controlling the directional control elements (e.g., the ailerons, elevators, and rudder) of airplanes have been developed. In recent years, fly-by-wire control systems have been developed to replace cable and hydraulic control systems in a variety of airplanes, including commercial and other jet transport airplanes. Regardless of the nature of the control system incorporated in an airplane, one problem associated, particularly with commercial jet airplanes, has been ride quality in the aft cabin of the airplane in the presence of wind gusts and atmospheric turbulence.
When flying in discrete wind gusts and atmospheric turbulence, the aft portion of the cabin of commercial and other transport airplanes experiences higher levels of lateral acceleration than do the forward and middle portions of the cabin. The increased level of lateral acceleration is not only uncomfortable from a passenger ride quality standpoint, but it can also create a difficult work environment for cabin crews of commercial jet airplanes, since the galley of many such airplanes is located in the aft cabin. The higher aft cabin lateral acceleration in response to discrete wind gusts and atmospheric turbulence of commercial jet airplanes is due to the rigid body weathercock stability and the flexible body bending of the fuselage of such airplanes. The principal cause of the higher aft cabin lateral acceleration is the lateral force produced by the lateral component of wind gusts and atmospheric turbulence impinging on the vertical stabilizer of the airplane. As is well known to those skilled in the art, aft body lateral acceleration can be significantly reduced by reducing or countering the effect of the vertical stabilizer of an airplane in wind gusts and atmospheric turbulence.
In the past, conventional modal suppression systems have been developed to reduce the effect of atmospheric turbulence and wind gusts in the aft cabin of commercial jet airplanes. Modal suppression systems employ lateral accelerometers to sense lateral side loads. The sensed information is used to generate rudder commands designed to attenuate the flexible body response to atmospheric turbulence and wind gusts. The signals produced by modal suppression systems are filtered to remove components of the signals lying outside of the flexible bending mode frequencies of the body (fuselage) of the airplane, namely the first body bending mode frequency, which lies between 2 and 4 Hz. Because modal suppression systems add damping to body oscillations after discrete wind gusts or atmospheric turbulence have excited the body to start such oscillations, modal suppression systems actually attenuate motion due to flexible coupling, rather than act to prevent such motion.
Recently, a method and apparatus has been developed for reducing undesired airplane aft cabin sideways motions resulting from atmospheric turbulence and wind gusts that uses a vertical stabilizer differential pressure sensor to produce a signal that is used to move the rudder on the vertical stabilizer in a direction that relieves vertical stabilizer loads. See U.S. Pat. No. 5,375,794, titled "Apparatus and Method for Reducing Aircraft Loads Resulting From Atmospheric Turbulence and Gusts," by Robert J. Bleeg (hereinafter '794 patent), the subject matter of which is incorporated herein by reference. The method and apparatus described in the '794 patent produces a rudder command that has been filtered to respond to wind gusts and air turbulence at and above the Dutch roll frequency range of an airplane. The filter has a corner frequency, f.sub.c, equal to 1 rad/sec. This corner frequency was selected to achieve the best balance between providing wind gust and atmospheric turbulence vertical stabilizer load alleviation and retaining good airplane handling qualities.
While the method and apparatus described in the '794 patent is a substantial improvement over modal suppression systems, it is subject to improvement, particularly when used in a fly-by-wire airplane control system. An improved method and apparatus of the general type described in the '794 patent is described in U.S. patent application Ser. No. 08/440,339, titled "Method and Apparatus for Reducing Unwanted Sideways Motion in the Aft Cabin and Roll-Yaw Upsets of an Airplane Due to Atmospheric Turbulence and Wind Gusts," by William F. Bryant et al. (Disclosure BOCO18417), filed concurrently herewith, the subject matter of which is incorporated herein by reference.
One of the potential problems associated with the apparatus and methods described in the '794 patent and in the foregoing patent application is that the rudder modification control provided by such methods and apparatus could cause rudder motion to exceed allowable structural limits in the event flow separation occurs where vertical stabilizer differential pressure is sensed. The present invention is directed to providing a method and apparatus for detecting airfoil flow separation and/or other aircraft conditions likely to produce airfoil flow separation and providing a control signal suitable for deactivating a selected control function, such as a rudder modification command produced by control systems of the type described in the '794 patent or the foregoing patent application.